Optimize memory usage

[gregberge]

• feat: accept a filepath or a buffer
• feat: optimize memory usage

[Copilot]

Pull request overview

This PR optimizes memory usage in the PNG fingerprinting library by eliminating intermediate buffer allocations and adds support for accepting file paths as input in addition to buffers.

Changes:

• Refactored fingerprint_diff to accept either a file path (string) or buffer as input
• Optimized PNG decoding to avoid unnecessary copying for RGBA8 format images
• Merged the hash computation directly into the grid density calculation to eliminate intermediate buffers (pack_2bit and fnv1a64 functions removed)

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/lib.rs	Added filepath/buffer input support, optimized RGBA8 decoding path, and merged packing/hashing into hash_grid_density to eliminate intermediate allocations
index.d.ts	Updated TypeScript signature to accept string | Buffer instead of just Buffer
test/index.spec.ts	Added test case to verify filepath input produces same fingerprint as buffer input

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